Connectors are used in a variety of electronic devices and many of these devices are data-transfer devices that are used to transmit data at data rates of 1 Gbps and higher. Cable assemblies are used to connect two or more devices together and it is desirable to maintain a continuity of impedance through the cable assembly and the mating interface because impedance mismatches and discontinuities can create signal reflections that generate noise. Electrical cables and printed circuit boards are examples of structures that can include single, continuous grounds in the form of, for example, a large ground plane in a circuit board or an extensive shield in a cable. One benefit of such a shared ground plane is that a common mode corresponding to each signal transmission line has a common voltage (e.g., the difference in potential between the ground associated with one signal pair and the ground associated another signal pair at a particular point along the transmission path is zero).
However, when a circuit board is coupled to a connector, the commonality of the ground structure is lost in the connector because each signal pair is usually associated with a different ground terminal within the connector. Because of different energies due to variations in the common mode for each signal path, when the various grounds are rejoined, they each will tend to have a different voltage and the differences will cause noise to be transmitted along the transmission line.
For example, many board connectors often include two grounds terminals that lie on opposing sides of a differential pair of signal terminals. Each ground terminal can have a different potential due to its position with respect to the differential pair. The potential on each ground terminal can also be affected based on the position of other signal pairs. As such, each ground terminal tends to have a different potential once separated. When these ground terminals are joined again (e.g., are terminated to a common ground plane), the difference in potential creates an energy wave that can reflect through the connector (creating noise on the signal transmission pairs). As the frequency of the energy wave created by the discontinuity in potential increases, the wave length of the frequency of the energy wave created by the voltage potential can approach the separated electrical length of the ground terminal, in which case the wave energy will tend to create a resonance that can significantly add to the noise level for the frequency of interest.
The resonance occurs in what is known as a “resonant cavity” and the boundaries of this resonant cavity can be equated with the separate electrical length of the terminal from a first point where the ground terminal is no longer associated with a single, continuous (e.g., shared) ground but is instead separate and thus can have a potential compared to other ground terminals. A region that forms the separate electrical length is where the potential imbalance occurs and this is also where resonance can occur. The noise resulting from the resonance leads to degradation in signal integrity. As can be appreciated, shortening the connector can also help shorten the electrical length, thus helping to diminish the effect of higher frequencies, but as frequencies increase, shortening the connector becomes less and less effective or practical. Therefore, certain people would appreciate connector system that could function at higher frequencies.